I. Field of the Invention
The present invention relates to a method and apparatus for auto-zeroing a flow sensor in a gas chromatograph while maintaining constant gas flow through the chromatograph and more particularly pertains to a two position, three-way integrated valve assembly for providing gradual, uninterrupted re-direction of gas flow from a flow measurement position to a bypass position wherein auto-zeroing of the flow sensor is accomplished.
II. Description of the Prior Art
In the practice of gas chromatography, accurate knowledge of the flow rate of the mobile phase is of critical importance. It is known in the art to measure the flow rate by means of flow sensors which are calibrated for specific carrier gasses. This calibration generally provides for a zero flow measurement. However, flow sensor output is subject to short term drift, which is largely temperature dependent, and long term drift, which is related to sensor fabrication techniques.
Methods and apparatus for correcting for sensor output drift are known in the prior art. One such method is disclosed in U.S. Pat. No. 5,542,286 to Wang et al. The input valves controlling the input into the chromatograph are shut, reducing the internal flow to zero. The indicated rate of flow is then measured using the flow sensor. If the value measured by the flow sensor during this test is different than the originally calibrated offset by some predetermined amount, then the newly measured value replaces the stored offset value. In a chromatograph where some minimum internal gas or liquid flow is necessary to prevent contamination of the instrument, a three-way valve can direct liquid or gas flow away from the flow sensor during the calibration run without eliminating internal flow through the chromatograph. The Wang et al. apparatus does not however provide for gradual, uninterrupted re-direction of gas flow from a flow measurement position to a bypass position and therefore suffers from pressure hammering which is a potential source of damage to the flow sensor or other components in the flow path.
It would therefore be desirable to provide a method and apparatus for auto-zeroing a flow sensor in a gas chromatograph which overcomes this deficiency in the prior art.